In general, a display apparatus includes an apparatus of a passive driving system such as a simple matrix, and an apparatus of an active matrix driving system in which a switching transistor is disposed for each pixel. In a liquid crystal display of an active matrix driving system, as shown in FIG. 16, a liquid crystal element 501 which also functions as a condenser and which includes a liquid crystal, and a transistor 502 which functions as a switching element are disposed for each pixel. In the active matrix driving system, when a pulse signal is inputted into a scanning line 503 by a scanning driver in a selection period to select the scanning line 503, and when a voltage for controlling transmittance of the liquid crystal is applied to a signal line 504 by a data driver, the voltage is applied to the liquid crystal element 501 via the transistor 502. In the liquid crystal element, liquid crystal molecules are oriented in a direction in accordance with the applied voltage to appropriately displace the transmittance of a light transmitted through the liquid crystal element. Even when the transistor 502 is brought in an off state in a non-selection period after the selection period, the liquid crystal element 501 functions as a condenser. Therefore, electric charges are held in accordance with a voltage value in an allowable range till the next selection period, and so the orientation direction of the liquid crystal molecules is maintained in the period. As described above, a liquid crystal display is a display apparatus of a voltage control system in which a voltage is newly written so as to obtain the light transmittance of the liquid crystal element 501 at a selection period time, and arbitrary gradation representation is performed in accordance with the voltage value.
On the other hand, the display apparatus in which an organic EL element is used as a self-luminous element does not require a backlight differently from the liquid crystal display, and is optimum for miniaturization. Moreover, there is not any restriction of a visual field angle differently from the liquid crystal display, and therefore practical use of the display apparatus for the next generation has largely been expected. Different from the liquid crystal element, the organic EL element emits the light by a current flowing inside. Therefore, an emission luminance does not directly depend on the voltage, and depends on current density.
From viewpoints of high luminance, contrast, and fineness, also in the organic EL display, there has been a demand especially for the active matrix driving system in the same manner as in the liquid crystal display. For the organic EL display, the current flowing in the selection period has to be increased in the passive driving system. On the other hand, in the active matrix driving system, an element for holding the voltages applied to opposite ends of the organic EL element is disposed for each pixel in order to maintain continuous emission of each organic EL element at a predetermined luminance so that the light is emitted even in the non-selection period. Therefore, the current value of the flowing current per unit time may be small. However, the organic EL element has only a remarkably small capacity as the condenser. Therefore, when the organic EL element is simply disposed instead of the liquid crystal element 501 in the circuit of the pixel shown in FIG. 16, it is difficult for the organic EL element to maintain the emission in the non-selection period.
To solve the problem, for example, as shown in FIG. 17, in the organic EL display of the active matrix driving system, an organic EL element 601 which emits the light at a luminance proportional to the current value of the current flowing inside, a transistor 602 which functions as a switching element, and a transistor 605 for passing a driving current through the organic EL element 601 in accordance with a gate voltage applied by the transistor 602 are disposed for each pixel. In this display, when the pulse signal is inputted into a scanning line 603 by a scanning driver in the selection period to select the transistor 605 connected to the scanning line 603, a signal voltage for passing a driving current having a predetermined current value through the transistor 605 is applied to a signal line 604 by the data driver. Then, the voltage is applied to a gate electrode of the transistor 605, and luminance data is written in the gate electrode of the transistor 605. Accordingly, the transistor 605 is brought into the on state, the driving current having a gradation in accordance with the voltage value applied to the gate electrode flows through the organic EL element 601 from a power via the transistor 605, and the organic EL element 601 emits the light at the luminance in accordance with the current value of the driving current. In the non-selection period after the selection period, even when the transistor 602 is in an off state, the electric charges continue to be held in accordance with a voltage between gate and source of the transistor 605 by a parasitic capacity between the gate and source of the transistor 605, and accordingly the driving current continues to be passed through the organic EL element 601. As described above, the driving current is principally controlled by the voltage value of the gate voltage of the transistor 605 outputted in the selection period to emit the light from the organic EL element 601 at a predetermined gradation luminance.
In general, for the transistor, a channel resistance depends on an ambient temperature, and the channel resistance changes by the use for a long time. Therefore, a gate threshold voltage changes with elapse of time, and the gate threshold voltage of each transistor in the same display region varies. Therefore, when the voltage value of the voltage applied to the gate electrode of the transistor 605 is controlled, the value of the current flowing through the organic EL element 601 is controlled. In other words, when a level of the voltage applied to the gate electrode of the transistor 605 is controlled, it is difficult to exactly control the luminance of the organic EL element 601.
To solve the problem, a technique of controlling the luminance by the current value of the current, not by the level of the voltage applied to the transistor has been researched. That is, instead of a voltage designating system in which the level of the gate voltage is designated in the signal line, a current designating system in which the current value of the current flowing through the organic EL element is directly designated for the signal line is applied to the active matrix driving system of the organic EL display.
However, in the organic EL display of the current designating system, the current value of the designated current is constant in the selection period when the designated current is passed. However, when the current value of the designated current is small, much time is required until the voltage is brought into a stationary state by the designated current. Therefore, the organic EL element does not emit the light at a desired luminance, and this results in a drop in display quality of the organic EL display.
On the other hand, when the selection period is lengthened, selection time becomes longer than a time for bringing the voltage into the stationary state. However, when the selection time lengthens, a display screen blinks. In this manner, the drop in the display quality of the organic EL display is caused.
Therefore, an advantage of the present invention is to perform high-quality display.